Add iTowns example

[aakaydin]

Target Use case

iTowns (@iTowns) is a web map rendering framework built on top of Three js. It would be nice to have a DeckGL integration and profit from data visualization capabilities

Proposed feature

Add a pure-js example showing how to add a DeckGL overlay to iTowns.

To Do List

• [X] Create an example using iTowns
• [X] Synchorinize the controls of both libraries
• [ ] Test